1. Field of the Invention
The present invention relates to a pressure sensor, a pressure measurement apparatus, a liquid ejection head and an image forming apparatus, and more particularly, to a pressure sensor, a pressure measurement apparatus, a liquid ejection head, and an image forming apparatus suitable for high-density mounting.
2. Description of the Related Art
An image forming apparatus is known which forms images on a recording medium, such as paper, by ejecting ink from nozzles toward the recording medium, while moving a liquid ejection head (inkjet head) having an arrangement of a plurality of nozzles, and the recording medium, relatively with respect to each other.
A known liquid ejection head mounted in the image forming apparatus is, for example, a piezo type liquid ejection head, in which ink is supplied to pressure chambers connected to nozzles, and the volume of the pressure chambers is changed, thereby causing the ink inside the pressure chambers to be ejected from the nozzles, by applying a drive signal corresponding to the image data to piezoelectric bodies forming pressure generating devices which are installed via a diaphragm plate on the pressure chambers.
On the other hand, there are also known thermal jet liquid ejection heads, which generate bubbles by heating the ink by means of heaters, or other heating elements, and eject ink droplets by means of the pressure thus generated.
Furthermore, a liquid ejection head provided with a pressure sensor is also known.
For example, Japanese Patent Application Publication No. 2000-94675 (and in particular, FIGS. 2 and 13) discloses technology for providing pressure sensors which measure the internal pressure of the pressure chambers, and performing feedback control on the basis of the measurement results of these pressure sensors, with the object of suppressing inadvertent ejection of satellite ink droplets. The pressure sensors include a one-piece piezoelectric sensor wafer which is relatively thin and covers all of the pressure chambers arranged in a one-dimensional fashion, and a plurality of long and thin piezoelectric sensor strips provided respectively for the pressure chambers.
In recent years, there have been demands to form high-quality images by increasing the number of pixels (number of dots) per unit surface area, by arranging a plurality of nozzles at high density. If a plurality of nozzles are arranged at high density in order to meet these demands, then the plurality of pressure chambers connected respectively to these nozzles are also required to be arranged at high density.
On the other hand, in order to ensure high-quality images, there is also a requirement to provide accurate determination of ejection defectiveness, such as ink ejection failures, and if pressure sensors are disposed respectively at the pressure chambers in order to meet such requirements, then the number of wires also increases in accordance with the number of pressure chambers. However, it is difficult to lay such wires, which increase in number in accordance with the number of pressure chambers, in the gaps between pressure chambers which are arranged at high density.
For example, if the pitch of a plurality of pressure chambers arranged in a two-dimensional configuration is 0.5 mm in both the longitudinal direction and the lateral direction, and if the size of each pressure chamber is 0.3 mm in both the longitudinal direction and the lateral direction, then each of the gaps between the pressure chambers in which the wires can be laid has a width of 0.2 mm (=0.5 mm−0.3 mm). Here, it is assumed that the desired image resolution is 2400 dots per inch (dpi), and that it is sought to arrange 48 pressure chambers (≧0.5 mm/(25.4 mm/2400)) in the longitudinal direction in order to achieve this resolution. In this case, one pressure sensor wire is required for each pressure chamber, and even if the total of 48 wires are extended in two different directions (for example, the rightward direction and the leftward direction), and even subtracting one wire of one pressure chamber that does not need to pass between pressure chambers, it is still necessary to pass 23 (=48/2−1) wires through each of the gaps (having a width of 0.2 mm as described above) between the pressure chambers. In this case, the wiring pitch is 8.7 μm (=0.2 mm/23), and if approximately one half of this wiring pitch is the actual width of the wires, then wiring is very difficult to achieve in practice.
Japanese Patent Application Publication No. 2000-94675 discloses a relatively thin one-piece pressure sensor which covers all of the pressure chambers. If the pressure amplitudes, which are different in the pressure chambers, are to be measured, then it is difficult to use the one-piece pressure sensor covering all of the pressure chambers, and hence it is necessary to use pressure sensors provided respectively for the pressure chambers, and wires are required to be provided for the pressure chambers, as described above, in respect of the pressure sensors provided correspondingly at the pressure chambers. Furthermore, Japanese Patent Application Publication No. 2000-94675 discloses pressure chambers arranged in a one-dimensional fashion; however, it makes no mention of pressure chambers arranged in a two-dimensional configuration, and makes no mention of pressure sensors for measuring the pressure respectively in the pressure chambers two-dimensionally arranged, and of wiring for such pressure sensors. In other words, it is difficult to arrange pressure chambers at high density in a two-dimensional configuration, as well as to measure the pressure in the pressure chambers.
Furthermore, if the surface area of the pressure sensors increases, their electrostatic capacitance also increases, and their output voltage can fall.